Many industries carry out gas/solids reactions that involve heat and mass transfer between the gas and solid phase materials. In most of these reactions, the solid material or particulate solids are retained as a final product. Therefore, if the reaction is carried out whereby the particulate solids are exposed to a flow of gas or the particulate solids are entrained in a flow of gas, the solids must thereafter be separated from the gas. The two principal operations, i.e. the reaction and the subsequent separation, have normally been considered and have been treated as mutually exclusive operations. The present invention provides an improvement over the prior art because it provides equipment and methods which effectively combine these two operations, i.e. the reaction and the separation, into a single vessel.
The consolidation of the reactor and separator within a single vessel saves space, construction costs and operating costs. In addition, the consolidation of the reaction and separation operations within a single vessel improves the mass transfer between the gas and solids and further enables the designers and operators of the apparatus to control the residence time of the particles within the reaction vessel.
By way of background, a variety of industries conduct continuous reactions involving gases and solids. The operations of drying, calcining, desulfurization and soil remediation are just a few examples. The operations of drying and calcining are similar in nature and provide clear examples of a gas/solids reaction.
"Drying" is defined as the removal of free or uncombined water from a raw feed stock, i.e., solid particulate. "Calcining", on the other hand, is defined as the removal of chemically bound water (or other gases) which produces a chemical change in the raw material. Both operations are conducted under high temperatures. Both operations involve the transfer of mass (water) from the solid particulate to the gas phase.
With specific reference to calcining, the most common calcining systems in use today are rotary kilns. In a rotary kiln, the solids move countercurrently to gas by gravity force as the kiln rotates. The rotation of the kiln enhances the mass transfer between the gas and solids. Some solids may be entrained in the gas flow and have to be collected by a separate collecting device. One primary disadvantage to the rotary kiln is that the residence time for all particles is the same, regardless of individual particle size. The uniform residence time results in an overexposure of smaller particles to the hot gas resulting in an over drying or "dead burning". Further, coarser or heavier particles may be under dried or under calcined.
A flash calciner includes a "cyclonic-type" reactor whereby the gas/solid flow obtains a cyclone patterned fluidized bed within the reactor. The solids are distributed in a cyclonic pattern against the reactor wall. This results in an uneven distribution of solids against the reactor wall thereby decreasing the mass transfer between the gas and solid phase. As in rotary kilns, the residence time for all particles in a flash calciner is the same.
Further, in both the rotary kiln and flash calciner designs, gas/solid separation is treated as a separate operation, downstream from the principal reaction.
A circulating fluidized bed reactor (CFB) includes a reaction chamber in communication with a separate cyclone which recirculates the solids back into the reaction chamber. While the cyclone of the CFB does separate out some of the solids, the CFB still requires a tall reaction chamber for proper solids distribution and a separate, bulky cyclone for recirculation of the solids back into the reaction chamber. Even if a CFB features an "integral" solids separator like the one disclosed in U.S. Pat. No. 4,679,511, a CFB includes high construction costs due to the required height of the reaction chamber and the separate cyclone.
Accordingly, many industries would welcome a reactor that combines a single reaction chamber with a gas/solids separation system. The principal gas/solids reaction could take place in the reaction chamber and a primary separation process would be conducted in the reaction chamber before the particulate solids product exits the system separately from the bulk of the gas. Further, many industries would welcome an improved reactor design that increases the mass transfer between the particulate solids and the gas and further many industries would welcome a reactor whereby the residence time of the particles is dependent upon particle size and is not uniform for particles of all sizes.